Secretion of the aqueous humour

Aqueous humor protein markers in myopia: a review

PURPOSE

Myopia is one of the most common forms of refractive error. Most myopia manifests itself as a relative growth of the eye axis, resulting in a state in which light is projected in front of the retina after being refracted by the refractive system of the eyeball. So far, the specific pathogenesis of myopia is still not well explained, through the results of animal experiments, researchers have proposed various possible scenarios, but all these are based on animal models, and there may still be a certain gap with the mechanism of true myopia in humans. The most readily available in clinical work is aqueous humor obtained during cataract surgery, for which we reviewed these studies of aqueous humor samples from myopic patients.

METHODS

A systematic literature search was done on PubMed using key words including "myopia," "aqueous humor," and "protein."

RESULTS

The results of existing aqueous humor studies have shown that the difference in substances in the aqueous humor of myopia is related to the degradation of the scleral matrix, chronic inflammation of the eye, pro-fibrosis, blood vessel production, and inhibition. There may be more than one reason associated with myopia progression.

CONCLUSION

The specific mechanism of myopia has not been fully elucidated. Therefore, the means of preventing and treating myopia should focus on inhibiting the degradation of the scleral matrix, promoting the proliferation of scleral collagen fibers, and alleviating chronic inflammation of the eyes. Further research into myopic aqueous humor may provide us with new insights.

Aqueous humor metabolite profile of pseudoexfoliation glaucoma is distinctive

Pseudoexfoliation (PEX) is a known cause of secondary open angle glaucoma. PEX glaucoma is associated with structural and metabolic changes in the eye. Despite similarities, PEX and primary open angle glaucoma (POAG) may have differences in the composition of metabolites. We analyzed the metabolites of the aqueous humor (AH) of PEX subjects sequentially first using nuclear magnetic resonance (1H NMR: HSQC and TOCSY), and subsequently with liquid chromatography tandem mass spectrometry (LC-MS/MS) implementing isotopic ratio outlier analysis (IROA) quantification. The findings were compared with previous results for POAG and control subjects analyzed using identical sequential steps. We found significant differences in metabolites between the three conditions. Principle component analysis (PCA) and partial least squares discriminant analysis (PLS-DA) indicated clear grouping based on the metabolomes of the three conditions. We used machine learning algorithms and a percentage set of the data to train, and utilized a different or larger dataset to test whether a trained model can correctly classify the test dataset as PEX, POAG or control. Three different algorithms: linear support vector machines (SVM), deep learning, and a neural network were used for prediction. They all accurately classified the test datasets based on the AH metabolome of the sample. We next compared the AH metabolome with known AH and TM proteomes and genomes in order to understand metabolic pathways that may contribute to alterations in the AH metabolome in PEX. We found potential protein/gene pathways associated with observed significant metabolite changes in PEX.

Aqueous Humor Markers in Retinoblastoma, a Review

Purpose

Retinoblastoma (Rb) is the most common primary intraocular cancer in children. Unlike with most solid tumors, direct biopsy is contraindicated due to risk of tumor dissemination. However, recent therapeutic techniques have allowed for the safe extraction of aqueous humor (AH) from eyes undergoing therapy, providing the unique opportunity to use AH as a liquid biopsy for Rb. Although the extraction of AH in Rb eyes undergoing therapy is new, the consideration of whether there are tumor biomarkers in the AH is not. The current manuscript is a systematic review of all studies that have examined biomarkers in the AH of Rb eyes. The authors hypothesized that AH sampling and analysis of tumor biomarkers may have new clinical relevance for the diagnosis, prognosis, and/or management of Rb.

Methods

A comprehensive database search (PubMed, Web of Science, Embase, and Cochrane Databases) was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement to identify articles on AH markers in Rb eyes. Inclusion criteria included English language articles with original reports on AH markers in the eyes of patients with confirmed Rb. Data on marker type, number of eyes, marker means and ranges, and when available, control values and clinicopathological correlations were collected. Articles were stratified based on marker type, and assessed quantitatively and qualitatively.

Results

An initial database search produced 325 articles, and an additional 11 articles were identified through searching citations. After removing duplicates and applying the eligibility criteria, we selected 27 articles to be included in the current review. A total of 463 eyes with histologically confirmed Rb were included in this review. The various markers and their values, with comparison to controls and clinicopathological correlations, are discussed.

Conclusions

AH sampling and tumor biomarker analysis in eyes without undergoing enucleation have the potential to revolutionize the management of Rb.

Translational Relevance

Although previous studies evaluated markers in the AH only after enucleation and not at diagnosis or during therapy, the clinical relevance of these markers was limited. However, recent changes in the management of Rb have allowed for safe sampling of the aqueous during therapy and, thus, correlation of tumor biomarkers with disease course. Thus, the authors felt it important to revisit previous research to evaluate whether these markers may now be applicable for the diagnosis, prognosis, or management of Rb

Presbyopia: Effectiveness of correction strategies

Presbyopia is a global problem affecting over a billion people worldwide. The prevalence of unmanaged presbyopia is as high as 50% of those over 50 years of age in developing world populations, due to a lack of awareness and accessibility to affordable treatment, and is even as high as 34% in developed countries. Definitions of presbyopia are inconsistent and varied, so we propose a redefinition that states "presbyopia occurs when the physiologically normal age-related reduction in the eye's focusing range reaches a point, when optimally corrected for distance vision, that the clarity of vision at near is insufficient to satisfy an individual's requirements". Strategies for correcting presbyopia include separate optical devices located in front of the visual system (reading glasses) or a change in the direction of gaze to view through optical zones of different optical powers (bifocal, trifocal or progressive addition spectacle lenses), monovision (with contact lenses, intraocular lenses, laser refractive surgery and corneal collagen shrinkage), simultaneous images (with contact lenses, intraocular lenses and corneal inlays), pinhole depth of focus expansion (with intraocular lenses, corneal inlays and pharmaceuticals), crystalline lens softening (with lasers or pharmaceuticals) or restored dynamics (with 'accommodating' intraocular lenses, scleral expansion techniques and ciliary muscle electrostimulation); these strategies may be applied differently to the two eyes to optimise the range of clear focus for an individual's task requirements and minimise adverse visual effects. However, none fully overcome presbyopia in all patients. While the restoration of natural accommodation or an equivalent remains elusive, guidance is given on presbyopic correction evaluation techniques.

[Influence of chemical compositions of anterior chamber aqueous humour and blood serum on the secretion of intraocular fluid]

PURPOSE

to identify the standard chemical composition of anterior chamber aqueous humour (AH) using energy dispersive spectrometer (EDS) integrated in scanning electron microscope (SEM); to study the influence of concentration gradient of chemical elements between aqueous humor and blood serum (BS) on the secretion of intraocular fluid (IOF) at normal and increased intraocular pressure (IOP).

MATERIAL AND METHODS

The study involved two groups of patients consisting of 33 people (33 eyes) each. The 1^st group included patients with cataract and normal IOP (control), the 2^nd group included patients with POAG and decompensated IOP. The samples (AH and BS) were taken during surgeries and studied using Oxford X-Max50 EDS integrated in EVO LS10 SEM.

RESULTS

The concentration ratios (mmol/l) of AH/BS for Na at normal IOP was 1.472/1.278, for K - 0.106/0.035. In cases with POAG with decompensated IOP, the concentration ratio of Na was 1.424/1.248, K - 0.114/0.036. The concentration ratio of N between AH in cases with decompensated IOP and AH of normotonic eyes was 1.151/0.960, for S - 0.020/0.012.

CONCLUSION

Consistent excess of Na and K concentrations in AH compared to BS indicates active participation of these osmotically active elements in the normal process of intraocular fluid secretion. The absence of significant differences in the ratios of Na and K in AH and BS at normal and decompensated IOP indicates low significance of these elements for pathological hypersecretion of intraocular fluid. Increased concentration of N in AH eyes with decompensated IOP compared with AH at normal IOP indicates possible involvement of nitrogen-containing osmotically active substances in the abnormal hypersecretion of intraocular fluid.

Phospholipid secretions of organ cultured ciliary body

Homeostasis of intraocular pressure (IOP) is important for the maintenance of anterior eye anatomic integrity, minimizing pressure-associated damage to the optic nerve, and maintaining a pressure gradient for blood flow to the eye. IOP is regulated by equilibrium between aqueous humor (AH) production and its outflow. The ciliary body (CB) is thought to actively secrete AH. However, whether AH composition and in particular, its phospholipids are entirely due to CB secretion remains uncertain. Comparison of phospholipids released by cultured CB, phospholipids present within CB tissue, within AH, and within blood and serum are consistent with release of most phospholipids into the AH by the CB. Treatment of CB in culture with timolol, a non-specific beta-adrenergic antagonist, alters the release of phospholipids by CB into the media. However, dorzalamide, a carbonic anhydrase inhibitor that reduces production of AH, does not affect phospholipid release thereby suggesting timolol, which also decreases IOP through decreased AH outflow, affects other physiological homeostatic mechanisms regulating aqueous outflow. These outflow changes also affect the composition of secreted phospholipids. We present evidence that release of lipids by the CB has a prolonged survival effect on cultured primary TM cells and TM tissue.

The exit strategy: Pharmacological modulation of extracellular matrix production and deposition for better aqueous humor drainage

Primary open angle glaucoma (POAG) is an optic neuropathy and an irreversible blinding disease. The etiology of glaucoma is not known but numerous risk factors are associated with this disease including aging, elevated intraocular pressure (IOP), race, myopia, family history and use of steroids. In POAG, the resistance to the aqueous humor drainage is increased leading to elevated IOP. Lowering the resistance and ultimately the IOP has been the only way to slow disease progression and prevent vision loss. The primary drainage pathway comprising of the trabecular meshwork (TM) is made up of relatively large porous beams surrounded by extracellular matrix (ECM). Its juxtacanalicular tissue (JCT) or the cribriform meshwork is made up of cells embedded in dense ECM. The JCT is considered to offer the major resistance to the aqueous humor outflow. This layer is adjacent to the endothelial cells forming Schlemm's canal, which provides approximately 10% of the outflow resistance. The ECM in the TM and the JCT undergoes continual remodeling to maintain normal resistance to aqueous humor outflow. It is believed that the TM is a major contributor of ECM proteins and evidence points towards increased ECM deposition in the outflow pathway in POAG. It is not clear how and from where the ECM components emerge to hinder the normal aqueous humor drainage. This review focuses on the involvement of the ECM in ocular hypertension and glaucoma and the mechanisms by which various ocular hypotensive drugs, both current and emerging, target ECM production, remodeling, and deposition.

Functional Anatomy of the Outflow Facilities

In order to understand the pathophysiology, select optimal therapeutic options for patients and provide clients with honest expectations for cases of canine glaucoma, clinicians should be familiar with a rational understanding of the functional anatomy of the ocular structures involved in this group of diseases. The topographical extension and the structural and humoral complexity of the regions involved with the production and the outflow of aqueous humor undergo numerous changes with aging and disease. Therefore, the anatomy relative to the fluid dynamics of aqueous has become a pivotal yet flexible concept to interpret the different phenotypes of glaucoma.

Aqueous humor phospholipids of DBA/2J and DBA/2J-Gpnmb(+)/SjJ mice

PURPOSE

To compare phospholipid profiles [phosphatidylcholine (PC), phosphatidylserine (PS), phosphatidylethanolamine (PE), and phosphatidylinositol (PI)] of normotensive and hypertensive aqueous humor (AH) from DBA/2J and compare them with phospholipid profiles of DBA/2J-Gpnmb(+)/SjJ mice.

METHODS

AH was obtained from young-normotensive DBA/2J, old -hypertensive DBA/2J mice, young and old DBA/2J-Gpnmb(+)/SjJ mice (aging control). Lipids were extracted using modified Bligh and Dyer method and subjected to mass spectrometric identification using appropriate class-specific lipid standards and ratiometric quantification. Corresponding aqueous phase (of extraction) protein concentrations were measured using Bradford method.

RESULTS

The total amount of phospholipids showed a decrease in the hypertensive state compared to normotensive state. The total PE and total PS contributed over 50% of the total amount. Total PS showed a remarkable decrease in hypertensive compared to normotensive state. In contrast, total PE in the hypertensive stage presented an increase in amount. Unique lipid species were found encompassing all four phospholipid classes in normotensive as well as in the hypertensive state. Several phospholipid species were found common to both states but with remarkable differences in amount in individual states. The ratio of lysophospholipids to total phospholipids is significantly reduced in the hypertensive state. Commensurate with reduced level of lysophospholipids, we found an increased level of lysophospholipase D (Autotaxin) in the hypertensive state. The difference of total phospholipids between young and old was 35.4% in DBA/2J group but 10% in DBA/2J-Gpnmb(+)/SjJ mice.

CONCLUSION

The significant change of phospholipids including lysophospholipids was found commensurate with the elevated intraocular pressure (IOP).

A contemporary concept of the blood-aqueous barrier

This review traces the evolution of the concept of the blood-aqueous barrier (BAB) during the past 20 years. The Classical model simply stipulated that the tight junctions of the iris vasculature and ciliary epithelium excluded plasma proteins from the aqueous humor (AH). It failed to reconcile the presence of AH protein levels equal to 1% of that found in plasma. Moreover, models of barrier kinetics assumed that the processes of AH secretion and plasma protein entry were directly linked. Thus, elevations of AH protein levels could only be explained by a pathological breakdown of the BAB. Over the last 20 years it has been shown that the plasma proteins in normal AH by-pass the posterior chamber entirely. Instead, these proteins diffuse from the capillaries of ciliary body stroma, into the iris stroma and then into the anterior chamber. This creates a reservoir of plasma-proteins in the iris stroma that is not derived from the iris vessels. This reservoir is prevented from diffusing posteriorly by tight junctions in the posterior iris epithelium. The one-way valve created by the pupil resting on the anterior lens capsule, combined with the continuous, forward flow of AH through the pupil, prevents protein reflux into the posterior chamber. Importantly, in the new paradigm, secretion of AH and the entry of plasma proteins into AH, are semi-independent events. This opens the possibility that AH protein levels could increase in the absence of breakdown of the BAB. Clinical consequences of this new paradigm of the BAB are discussed.

Analysis of aqueous humour proteins in patients with retinoblastoma

BACKGROUND

To investigate aqueous humour protein composition from retinoblastoma patients.

DESIGN

Prospective, hospital-based study.

PARTICIPANTS

Eighteen retinoblastoma patients (Reese-Ellsworth stage V or ABC classification group E RB) undergoing ocular enucleation, and 10 normal subjects undergoing cataract surgery. Five of 18 patients presented with associated secondary glaucoma whereas 13 had no secondary glaucoma; 5 of 13 patients with no secondary glaucoma received chemotherapeutical treatment with melphalan.

METHODS

Aqueous humour samples were collected by limbal paracentesis of the anterior chamber after ocular enucleation in patients and after the stab peripheral corneal incision in controls. Total protein concentration according to Bradford method and sodium dodecyl sulphate-polyacrylamide gel electrophoresis of the samples were performed.

MAIN OUTCOME MEASURE

Aqueous humour protein concentration.

RESULTS

Aqueous humour protein concentration was significantly higher in retinoblastoma patients than controls (P < 0.01); patients with secondary glaucoma presented the highest values (P < 0.05 vs. controls); patients treated with melphalan presented a significant decrease (P < 0.01) versus non-treated; controls did not significantly differ from treated patients. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis pattern in retinoblastoma patients who did not receive any treatment was very different either from treated or from controls.

CONCLUSION

This study represents a preliminary step towards a more accurate two dimensional electrophoresis (2DE) pattern, which will be combined with mass spectrometry analysis to clarify the potential role of specific proteins in tumour development and progression; although these results suggest that aqueous humour protein pattern in retinoblastoma is characteristic, several aspects of the study are still under investigation.

Pooling aqueous humor samples: bias in 2D-LC-MS/MS strategy?

The proteomic analysis of body fluids presents a major challenge in studies of human diseases. Traditional techniques for protein separation require large volumes and large amounts of protein, which may be difficult to obtain for certain fluids, such as the aqueous humor (AH). Two-dimensional liquid chromatography (2D-LC-MS/MS), adapted for peptides separation from complex protein mixtures, provides an alternative approach in proteomic analysis with a potential utility in biomarker research. We investigated several different 2D-LC-MS/MS methods for use with the AH of patients with cataract, traditionally used as a control group in studies of ocular diseases. We compared analyses of individual samples with analyses of pools of proteins or peptides, and found that the investigation strategy used strongly influenced protein identification. We identified 71 proteins related to extracellular proteins highly abundant in serum (e.g., albumin or transferrin) and involved in various functions, such as transport and metabolism, together with intracellular (myeloblastin) or organelle-specific proteins (cytochrome c). An evaluation of the advantages and disadvantages of each method suggested that individual analyses and the use of peptide mixtures should be favored as complementary techniques in the search for biomarkers in ocular diseases.

Depression of intraocular pressure following inactivation of connexin43 in the nonpigmented epithelium of the ciliary body

PURPOSE

Conditional inactivation of connexin43 (Cx43) in the pigmented epithelium of the mouse eye results in a reduction in aqueous humor production and complete loss of the vitreous chamber. It was proposed that gap junctions between pigmented and nonpigmented epithelia of the ciliary body are critical for the production of the aqueous humor. To form such junctions, Cx43 in the pigmented epithelium must interact with connexin(s) present in the adjacent cells of the nonpigmented epithelium. The importance of Cx43 expression in the nonpigmented epithelium for the establishment of gap junctions and the regulation of intraocular pressure was tested.

METHODS

To inactivate Cx43 in the nonpigmented epithelium of the mouse eye, a mouse line was crossed with a floxed Cx43 locus (Cx43(flox/flox)) and a transgenic mouse line expressing cre recombinase under the control of the Pax6alpha promoter. General eye structure was evaluated by light microscopy, gap junctions were analyzed by electron microscopy, and intraocular pressure was directly assessed with micropipettes.

RESULTS

In Pax6alpha-cre/Cx43(flox/flox) mice, Cx43 was partially inactivated in the nonpigmented epithelium of the ciliary body and iris. Animals developed dilatations between the pigmented and nonpigmented epithelia and displayed a significant reduction in intraocular pressure. However, gap junctions between the ciliary epithelial layers were decreased but not eliminated.

CONCLUSIONS

Cx43 expression in the nonpigmented epithelium of the ciliary body contributes to the formation of gap junctions with the cells of the pigmented epithelium. These gap junctions play a critical role in maintaining the physical integrity of the ciliary body epithelium. Although the partial loss of Cx43 from the nonpigmented epithelium was correlated with a measurable drop in intraocular pressure, possible changes in Cx43 in the aqueous outflow pathway may provide an additional contribution to the observed phenotype.

Sympathetic nervous system modulates the ocular hypotensive action of MT2-melatonin receptors in normotensive rabbits

The aim of this study was to investigate the hypotensive effect of the melatonin analogue, N-butanoyl-2-(2-methoxy-6H-isoindolo[2,1-a]indol-11-yl)ethanamine (IIK7), through MT(2)-melatonin receptors and the involvement of the sympathetic nervous system in this action in New Zealand rabbit eyes. The topical application of melatonin or IIK7 produced a reduction in intraocular pressure of 20.2 +/- 5.3% and 38.5 +/- 3.2% respectively. This effect was concentration-dependent; it was blocked by selective MT(2) receptor antagonists and was severely diminished after chemical sympathectomy. Immunohistochemistry and western blot analysis showed the ciliary processes as the site of this action and no co-localization of MT(2)-melatonin receptor with the sympathetic nervous system was observed. The beta-adrenergic agonists, terbutaline and salbutamol, potentiated the hypotensive effect of IIK7 reducing intraocular pressure (IOP) 41.75 +/- 4.26% and 44.7 +/- 5.6% respectively. Also, IIK7 in presence of the nonspecific protein phosphatase inhibitor okadaic acid, lowered IOP 32.2 +/- 4.5% and in presence of forskolin plus 3-isobutyl-1-methylxanthine decreased IOP in 32.2 +/- 5.47%. These data suggest that the melatonin agonist IIK7 reduces intraocular pressure by acting through MT(2)-melatonin receptors presumably decreasing aqueous humour formation. Also, in the presence of beta-adrenoceptor agonists MT(2)-melatonin receptors activity increase their ability to reduce IOP.

Study on the ocular pharmacokinetics of ion-activated in situ gelling ophthalmic delivery system for gatifloxacin by microdialysis

The poor bioavailability and therapeutic response exhibited by conventional ophthalmic solutions due to rapid precorneal elimination of the drug may be overcome by the use of gel system. The present work was conducted to evaluate the relative bioavailability of ion-activated in situ ophthalmic gel of gatifloxacin by microdialysis. The conventional ophthalmic solution of gatifloxacin was used as reference. The AUC of test group is 3.8-fold vs. the reference group (1.4316 +/- 0.1327 microg.mL(-1).h vs. 0.3756 +/- 0.0380 microg.mL(-1).hr) (P < 0.05), and the C(max) of test group vs. the control group is 3.0-fold (0.3363 +/- 0.0634 microg.mL(-1) vs. 0.1112 +/- 0.0151 microg.mL(-1)) (P < 0.05). The T(max) of test group is longer than that of reference group (2.0 +/- 0.67 hr vs. 0.667 +/- 0.17 hr) (P < 0.1), and K(e) of test group is lower than that of reference group. The developed formulation has a higher bioavailability and longer residence time in aqueous humor than conventional ophthalmic solutions. The developed system is a viable alternative to conventional eye drops.

Fluid transport phenomena in ocular epithelia

This article discusses three largely unrecognized aspects related to fluid movement in ocular tissues; namely, (a) the dynamic changes in water permeability observed in corneal and conjunctival epithelia under anisotonic conditions, (b) the indications that the fluid transport rate exhibited by the ciliary epithelium is insufficient to explain aqueous humor production, and (c) the evidence for fluid movement into and out of the lens during accommodation. We have studied each of these subjects in recent years and present an evaluation of our data within the context of the results of others who have also worked on electrolyte and fluid transport in ocular tissues. We propose that (1) the corneal and conjunctival epithelia, with apical aspects naturally exposed to variable tonicities, are capable of regulating their water permeabilities as part of the cell-volume regulatory process, (2) fluid may directly enter the anterior chamber of the eye across the anterior surface of the iris, thereby representing an additional entry pathway for aqueous humor production, and (3) changes in lens volume occur during accommodation, and such changes are best explained by a net influx and efflux of fluid.

Hyposmotic activation of ICl,swell in rabbit nonpigmented ciliary epithelial cells involves increased ClC-3 trafficking to the plasma membrane

In mammalian nonpigmented ciliary epithelial (NPE) cells, hyposmotic stimulation leading to cell swelling activates an outwardly rectifying Cl(-) conductance (I(Cl,swell)), which, in turn, results in regulatory volume decrease. The aim of this study was to determine whether increased trafficking of intracellular ClC-3 Cl channels to the plasma membrane could contribute to the I(Cl,swell) following hyposmotic stimulation. Our results demonstrate that hyposmotic stimulation reversibly activates an outwardly rectifying Cl(-) current that is inhibited by phorbol-12-dibutyrate and niflumic acid. Transfection with ClC-3 antisense, but not sense, oligonucleotides reduced ClC-3 expression as well as I(Cl,swell). Intracellular dialysis with 2 different ClC-3 antibodies abolished activation of I(Cl,swell). Immunofluorescence microscopy showed that hyposmotic stimulation increased ClC-3 immunoreactivity at the plasma membrane. To determine whether this increased expression of ClC-3 at the plasma membrane could be due to increased vesicular trafficking, we examined membrane dynamics with the fluorescent membrane dye FM1-43. Hyposmotic stimulation rapidly increased the rate of exocytosis, which, along with ICl,swell, was inhibited by the phosphoinositide-3-kinase inhibitor wortmannin and the microtubule disrupting agent, nocodazole. These findings suggest that ClC-3 channels contribute to I(Cl,swell) following hyposmotic stimulation through increased trafficking of channels to the plasma membrane.

Cl− concentrations of bovine, porcine and ovine aqueous humor are higher than in plasma

Aqueous humor (AH) Cl- levels were measured from live cows and pigs to confirm the Cl(-)-secretory activities found across the ciliary bodies of these species in in vitro experiments. The values obtained were compared to those of the electrolyte in plasma (PL). In addition, Na+ and K+ concentrations were also measured in AH and PL. For analogy, the same measurements were made in sheep and rabbit. The results indicate that the Cl- AH-to-PL ratios of the bovine, porcine and ovine species, but not that of rabbit, are comparable to published values for humans. Thus, the transport activities of the ciliary epithelia of the former animals may be more adequate as a representative model for human than the commonly used rabbit, which exhibits strikingly different transport properties with a ciliary epithelium that is primarily a bicarbonate-transporting tissue.

PD128,907 induces ocular hypotension in rabbits: involvement of D2/D3 dopamine receptors and brain natriuretic peptide

The purpose of this study was to determine the potential role of brain natriuretic peptide (BNP) in the PD128,907 (a dopamine D2/D3 receptor agonist)-induced ocular hypotension in rabbits. The effects of topical application of PD128,907 (75, 250, 750 microg) on intraocular pressure (IOP) were investigated. The lowest dose (75 microg) did not alter IOP; while the higher doses (250 and 750 microg) reduced IOP bilaterally. The PD128,907 (250 microg)-induced ocular hypotension, which lasted 3 hours, could be blocked by raclopride (1000 microg), a dopamine D2/D3 receptor antagonist, as well as by sympathetic denervation. Aqueous humor inflow was reduced by intravitreal injection of PD128,907 (10 microg) by 67% at 1 and 2 hours, which then returned to baseline at 3 hours. Furthermore, topical application of PD128,907 (250 microg) elevated aqueous BNP levels by 3-fold at 30 minutes, 6-fold at 1 hour and 5-fold at 2 hours, which could be blocked by pretreatment with raclopride (250 microg). Taken together, PD128,907-induced ocular hypotension by activation of dopamine D2/D3 receptors. This action was associated with reduced aqueous humor inflow and increased aqueous BNP levels.

Purinergic signaling in the rabbit ciliary body epithelium

In the past forty years, a wealth of information has accumulated that points to the presence of adenosine and adenine nucleotides in the anterior segment of the eye and a number of hypotheses have been introduced to describe the possible role of these agents in the regulation of aqueous humor flow. However, in the absence of a generally accepted model for the cellular and molecular mechanisms of aqueous humor formation by the ciliary body epithelium, efforts to identify the signal transduction pathway(s) responsible for regulation of the ion and water transport have not been successful. This article briefly reviews the evidence for (i). the presence in aqueous humor of adenine nucleotides, cyclic adenosine monophosphate and adenosine, their metabolic product, (ii). the possible role of these agents in the regulation of aqueous humor dynamics, and (iii). the expression of ecto-nucleotidases, receptors, and second messengers that may mediate such regulation. Finally, a model for the regulation of aqueous humor formation by adenosine and ATP is proposed.

Molecular mechanisms of water transport in the eye

The four major sites for ocular water transport, the corneal epithelium and endothelium, the ciliary epithelium, and the retinal pigment epithelium, are reviewed. The cornea has an inherent tendency to swell, which is counteracted by its two surface cell layers, the corneal epithelium and endothelium. The bilayered ciliary epithelium secretes the aqueous humor into the posterior chamber, and the retinal pigment epithelium transports water from the retinal to the choroidal site. For each epithelium, ion transport mechanisms are associated with fluid transport, but the exact molecular coupling sites between ion and water transport remain undefined. In the retinal pigment epithelium, a H+-lactate cotransporter transports water. This protein could be the site of coupling between salt and water in this epithelium. The distribution of aquaporins does not suggest a role for these proteins in a general model for water transport in ocular epithelia. Some water-transporting membranes contain aquaporins, others do not. The ultrastructure is also variable among the cell layers and cannot be fitted into a general model. On the other hand, the direction of cotransport in symporters complies with the direction of fluid transport in both the corneal epi- and endothelium, as well as the ciliary epithelium and retinal pigment epithelium.

Similarity of A(3)-adenosine and swelling-activated Cl(-) channels in nonpigmented ciliary epithelial cells

Chloride release from nonpigmented ciliary epithelial (NPE) cells is a final step in forming aqueous humor, and adenosine stimulates Cl(-) transport by these cells. Whole cell patch clamping of cultured human NPE cells indicated that the A(3)-selective agonist 1-deoxy-1-(6-[([3-iodophenyl]methyl)amino]-9H-purin-9-yl)-N-methyl-be ta-D-ribofuranuronamide (IB-MECA) stimulated currents (I(IB-MECA)) by approximately 90% at +80 mV. Partial replacement of external Cl(-) with aspartate reduced outward currents and shifted the reversal potential (V(rev)) from -23 +/- 2 mV to -0.0 +/- 0.7 mV. Nitrate substitution had little effect. Perfusion with the Cl(-) channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and niflumic acid inhibited the currents. Partial Cl(-) replacement with aspartate and NO(3)(-), and perfusion with NPPB, had similar effects on the swelling-activated whole cell currents (I(Swell)). Partial cyclamate substitution for external Cl(-) inhibited inward and outward currents of both I(IB-MECA) and I(Swell). Both sets of currents also showed outward rectification and inactivation at large depolarizing potentials. The results are consistent with the concept that A(3)-subtype adenosine agonists and swelling activate a common population of Cl(-) channels.

Effect of SR121463, a selective non-peptide vasopressin V2 receptor antagonist, in a rabbit model of ocular hypertension

The activity on intraocular pressure (IOP) of SR121463, a selective non-peptide arginin-vasopressin (AVP) V2 receptor antagonist, was investigated in a rabbit model of ocular hypertension. We first demonstrated that, in vitro, SR121463 displayed high competitive affinity for rabbit vasopressin V2 receptors (Ki = 2.1 +/- 1.2 nM). In vivo, SR121463 was instilled once (at concentrations ranging from 0.1 to 3%), or for 10 days (20 instillations) at 1% concentration, in the eye of ocular hypertensive rabbits (intraocular injection of 0.14 mg alpha-chymotrypsin). SR121463 also was instilled at 1% in the normotensive eye or intravenously injected (100 microg/kg) to ocular hypertensive rabbits. SR121463 was compared to timolol 0.5% or to clonidine 0.25%. Additionally, local and systemic safety aspects were examined. Results showed that SR121463 was locally well-tolerated and had no anesthetic effect. A significant decrease in IOP of the hypertensive eye was observed for concentrations of SR121463 > or =1%. This decrease was comparable to that obtained with reference compounds. A similar activity was found after intravenous administration. No tachyphylaxis was observed after 10 days, and no contralateral or systemic effect was noted. Also, when applied on the normotensive eye or when intravenously injected, SR121463 had no effect on the normotensive eye. These results on IOP and the good local and systemic safety profile, suggest that a potent vasopressin V2 receptor antagonist, SR121463, could be of value for the treatment of glaucoma, through a mechanism of action that remains to be elucidated.

Tamoxifen and ATP synergistically activate Cl- release by cultured bovine pigmented ciliary epithelial cells

Purines alter aqueous humour secretion by the bilayered ciliary epithelium. Adenosine but not ATP shrinks non-pigmented ciliary epithelial (NPE) cells by activating Cl- channels. We now report effects of ATP on pigmented ciliary epithelial (PE) cells. Cultured bovine PE cells were studied volumetrically by electronic cell sorting. ATP and tamoxifen acted synergistically to shrink PE cells. Neither ATP nor tamoxifen alone had a consistent effect on cell volume. The tamoxifen, ATP-activated shrinkage required Cl- release since the response was blocked by removing Cl- and was inhibited by the Cl- channel blockers 5-nitro-2-(3-phenylpropylamino)-benzoate and 4,4'-diisothiocyano-2,2'-disulfonic acid. The modulating effect of tamoxifen could have reflected many actions of tamoxifen. Our data do not support the suggestion that tamoxifen inhibits protein kinase C (PKC) or calcium-calmodulin, or that it acts on histamine or carbachol receptors. The shrinkage produced by ATP and tamoxifen was blocked by 17beta-oestradiol, but not 17alpha-oestradiol. The cooperative interaction between tamoxifen and ATP was not mediated by an enhanced rise in [Ca2+]i. The results indicate that tamoxifen interacts synergistically with ATP to activate Cl- release by the PE cells.

Pharmacodynamics of beta-blocker modulation of aqueous humor production

A conscious rabbit model with microdialysis sampling of endogenous aqueous humor ascorbate was developed in order to assess the pharmacodynamics of beta-blocker modulation of aqueous humor production. CMA/20 microdialysis probes were implanted in the anterior chamber of each eye of rabbits (n = 6). After a 2 week recovery period, an i.v. bolus of 14C-ascorbate (20 microCi) was administered. Blood samples and aqueous humor microdialysis probe effluent were collected and analysed for endogenous and 14C-ascorbate to estimate the basal rate of ascorbate blood to aqueous humor secretion (Ro). After a 1 hr washout, each rabbit received a series of three doses of 3H-propranolol (750-3000 microg, 16.5 microCi mg(-1)) every 60 min into the lower cul-de-sac of each eye. Probe effluent was analysed for endogenous ascorbate and 3H-propranolol; ascorbate and propranolol in the iris/ciliary body, vitreous and aqueous was determined at the end of the experiment. Nonlinear least-squares regression analysis of the concentration-time profiles for aqueous humor ascorbate was performed to estimate the change in aqueous humor flow. The average basal aqueous humor ascorbate secretion rate was approximately 48/microg hr(-1). Propranolol (1500 microg) produced significant increases in aqueous humor ascorbate, this observation is consistent with a reduction in aqueous humor production (approximately 47%). Analysis of intraocular tissue ascorbate indicated that propranolol inhibited ascorbate secretion at the 3000 microg dose, the highest dose examined in this study; this inhibition was not observed at the 750 microg or 1500 microg doses. Changes in aqueous humor production precipitated by the administration of beta-adrenergic antagonists can be estimated by measuring changes in aqueous humor ascorbate concentrations in the conscious rabbit. Microdialysis sampling of aqueous humor for endogenous ascorbate provides a relevant analytic tool to estimate modulatory effects of anti-glaucoma drugs on aqueous humor production.

Formation of the aqueous humor

1. Glaucoma is a worldwide disease affecting approximately 1-2% of the population aged over 35 years in industrial countries and is a major cause of blindness. 2. Glaucoma is usually associated with an increased intraocular pressure reflecting an imbalance between the rate of production of fluid (the aqueous humor) by the ciliary epithelial cells and its drainage from the eye. Therefore, it is important to understand how this secretion is produced. This requires a knowledge of ciliary epithelial cell composition, which has, in the past, proved difficult to obtain in mammalian preparations. 3. We have recently used the technique of electron-probe X-ray microanalysis to determine this composition under a variety of in vitro conditions. 4. Our results have led to a new model for this secretion that emphasizes the potential secretory role of the Na+/K+/2Cl- cotransporter.

Differential gene expression in the human ciliary epithelium

The generation of expression and subtractive libraries from the ocular ciliary body and cultured ciliary epithelial cells has been instrumental in the cloning, identification and characterization of many genes which, overall reflect a representative profile of transcripts expressed in ciliary nonpigmented, ciliary pigmented and ciliary muscle cells. The cell-specific expression of some of these genes (i.e. a neurotrophic factor, a gene associated with juvenile open glaucoma, and a visual component) reveal a degree of cell differentiation with a diversity of functions and properties higher than previously thought. The protection from light-induced oxidative reactions, free radicals and detoxification, may be partially attributed to the high level of expression in the ciliary epithelium of antioxidative enzymes (i.e., glutathione S-transferase, glutathione peroxidases, selenoprotein-P). The expression of genes encoding plasma proteins (i.e., complement component C4, alpha2-macroglobulin, apolipoprotein D) is in contrast with the view that plasma proteins in aqueous humor are synthesized outside the eye (i.e., liver). The identification of neuropeptide-processing enzymes (i.e., prohormone convertases, carboxypeptidase E, peptidyl-glycine-alpha-amidating monoxigenase), neuropeptides (i.e., secretogranin II, neurotensin) and regulatory peptides (i.e., atrial natriuretic peptide and angiotensinogen) with hypertensive and hypotensive activities provide the molecular basis to support the view that the ciliary epithelium is a neuroepithelium with neuroendocrine functions. We propose a working model to demonstrate that aqueous humor and intraocular pressure are under neuroendocrine control through regulatory peptides synthesized and released by the ciliary epithelium and targeting the peptide producing cells at the inflow system by an autocrine mechanism and/or cells at the outflow system (i.e., trabecular meshwork cells) by a paracrine mechanism. Finally, we hypothesize that these mechanisms could be entrained in the light-dark cycle following the circadian rhythm of aqueous humor and intraocular pressure.

Microdialysis evaluation of the ocular pharmacokinetics of propranolol in the conscious rabbit

PURPOSE

This study was conducted to assess the effects of anesthesia and aqueous humor protein concentrations on ocular disposition of propranolol.

METHODS

Rabbits were anesthetized and a microdialysis probe was inserted into the anterior chamber of one eye; the contralateral eye served as a control. At timed intervals after probe placement, a 100-microl sample of aqueous humor was aspirated from each eye to determine protein concentration. In vitro protein binding parameters were used to simulate the impact of protein concentration on propranolol disposition. To assess the influence of anesthesia, probes were implanted in the anterior chamber of each eye. After >5-day stabilization, conscious and anesthetized rabbits (n = 3/group) received a 200-microg topical dose of [3H]DL-propranolol in each eye; propranolol was assayed in probe effluent.

RESULTS

Changes in aqueous humor protein concentrations were observed following probe insertion. Simulations demonstrated that the unbound propranolol AUC (approximately 2.4-fold) in aqueous humor should be reduced due to protein influx. Intraocular propranolol exposure in anesthetized rabbits was approximately 8-fold higher than in conscious rabbits, and approximately 1.9-fold higher than in rabbits without a post-surgical recovery period.

CONCLUSIONS

Anesthesia and time-dependent aqueous humor protein concentrations may alter ocular pharmacokinetics, and must be taken into account in the design of microdialysis experiments.

Glycoconjugates of the rat ciliary body epithelium: a lectin histochemical and protein blotting study

The present study sought to identify and partially characterize the glycoconjugates specific to the double-layered ciliary body epithelium of the rat eye by lectin histochemistry and lectin blottings. Hydrated paraffin sections of Carnoy-fixed Sprague-Dawley rat eyes were stained with a panel of 21 different biotinylated lectins, followed by streptavidin-peroxidase and the glucose oxidase-diaminobenzidine-nickel staining procedure. The results of lectin histochemistry revealed that the inner epithelial layer was rich in GlcNAc(beta1,4)GlcNAc, alpha-Gal, Gal(beta1,3)GalNAc, GalNAc(alpha1,3)GalNAc/Gal, GalNAc(alpha1,6)Gal, Fuc(alpha1,2)Gal(beta1,4)GlcNAc and Gal(beta1,4)GlcNAc(beta1,2)Man(alpha1,6) sugar residues as shown by its positive reactivities with S-WGA, PWA, DSA, GS-I-B4, PNA, DBA, SBA, WFA, UEA-I, LTA and PHA-E. The reactivities of GS-I-B4, PNA, DBA and SBA were restricted to the inner layer at the tips of the ciliary processes. On the other hand, the outer epithelial layer was stained evenly by DSA and Jacalin, and partly by MAA, showing that this epithelial layer was rich in GlcNAc(beta1,4)GlcNAc, Gal(beta1,3)GalNAc and NeuAc(alpha2,3)Gal disaccharides. These lectin binding patterns of the ciliary body epithelium suggest a topographical and functional difference in this double cell-layered epithelium. Their possible roles in the secretion of aqueous humour and production of ciliary zonule are discussed. Some identified lectin markers specific to these two cell layers may be useful for further experimental studies. Glycoproteins extracted from the dissected ciliary body were separated by SDS-PAGE electrophoresis and analyzed by protein blottings with 8 different lectins. The results showed that at least 10 major membrane-bound glycoproteins, with molecular weights ranging from 30 to 150 kD, rich in beta-GlcNAc, beta-Gal, alpha/beta-GalNAc and NeuAc(alpha2,6)Gal residues, were present in the microsomal fraction.

pICln can regulate swelling-induced Cl- currents in either layer of rabbit ciliary epithelium

Swelling-induced Cl- currents were investigated in freshly prepared non-pigmented epithelial (NPE) and pigmented epithelial (PE) cells of the rabbit ciliary body using the whole-cell patch clamp technique. Exposure of both NPE and PE cells to hypotonic stress induced Cl- currents that exhibited outward rectification and were insensitive to Ca+2. We found that swelling-induced Cl- currents in PE cell are observed shortly after isolation. The swelling-induced Cl- current showed little or no inactivation at positive membrane voltages and was sensitive to 100 microM NPPB and 100 microM DIDS. Injection of cRNA encoded rabbit pICln into Xenopus oocytes produced an outwardly rectifying Cl- current displaying features consistent with the swelling-induced Cl- current in epithelium. pICln is ubiquitous in the ciliary epithelium. It participates in the equilibration of short term tonicity alterations, a phenomenon underlying mechanisms with larger and slower amplitudes for aqueous secretion by these cells.

Evaluation of microdialysis sampling of aqueous humor for in vivo models of ocular absorption and disposition

The dynamics of beta-adrenergic-associated reductions in aqueous humor production for treatment of elevated intraocular pressure are not well understood. In particular, the relationship between ocular pharmacokinetics and pharmacodynamics has yet to be established. This study was undertaken to develop a procedure for examining the ocular absorption and disposition of topically administered ophthalmic beta-adrenergic antagonists in individual animals. Dogs were anesthetized with isoflurane and a microdialysis probe was implanted in the anterior chamber of one eye and perfused with 0.9% saline at a rate of 2 microliters min-1. 3H-propranolol was administered by intracameral injection or topically. Each dog received intracameral and topical propranolol, in alternate eyes on separate days, in a randomized cross-over fashion. Microdialysis probe effluent was collected every 5 min for > or = 2.5 h; concentrations of propranolol were determined by liquid scintillation spectroscopy and were corrected for probe recovery of the substrate as determined by in vivo retrodialysis (approximately 46%) to estimate aqueous humor concentrations. In separate experiments in rabbits, microdialysis probes were implanted in each eye. 3H-propranolol was administered topically to one eye; the contralateral eye received intracameral 3H-propranolol. Model-independent pharmacokinetic parameters for each treatment phase were calculated. The mean +/- S.D. times to peak concentration of propranolol in aqueous humor were 86.6 +/- 47.6 min in the dog and 54.1 +/- 20.4 min in the rabbit. The terminal rate constant was 0.0189 +/- 0.00429 min-1 in the dog vs. 0.00983 +/- 0.00546 min-1 in the rabbit. Intraocular tissue availability of propranolol differed markedly between the dog (n = 3) and rabbit (n = 3) (approximately 0.056 in the dog vs. approximately 0.55 in the rabbit). These results demonstrate the utility of microdialysis sampling for examination of ocular pharmacokinetics.

Cloning and functional expression of a swelling-induced chloride conductance regulatory protein, plCln, from rabbit ocular ciliary epithelium

The cDNA encoding a swelling-induced chloride conductance regulatory protein, plcln, was cloned from rabbit ciliary epithelium by using a polymerase chain reaction (PCR)-based approach. The open reading frame encoding 236 amino acids possesses high amino acid identity (93/%) with the previously cloned plcln from human ciliary epithelium. Outwardly rectifying currents were recorded in Xenopus oocytes injected with plcln cRNA, a result consistent with plcln expression in ciliary epithelium. A widespread distribution and marked expression of plcln mRNA in both nonpigmented ciliary epithelial (NPE) cells and pigmented ciliary epithelial (PE) cells was found for the first time. In situ hybridization analysis showed that plcln expression is more abundant in NPE than PE. These findings are consistent with the idea that plcln may be an important regulatory element in these secretory cells.

Gene expression of proteases and protease inhibitors in the human ciliary epithelium and ODM-2 cells

Complementary DNAs (cDNAs), corresponding to the human proteinases cathepsins D and O and proteinase inhibitors alpha2-macroglobulin and PP5/TFPI-2, have recently been isolated and identified from a subtractive human ciliary body library. In the present study we determined: (i) their pattern of expression in the human eye; (ii) the ability of the ciliary body and/or ciliary epithelial cells to synthesize and secrete cathepsin D and alpha1-antitrypsin in vitro; and (iii) whether alpha1-antitrypsin expression in cultured ciliary epithelial cells is modulated by protein kinase C activation. Northern analysis demonstrated that the ciliary body expresses high levels of cathepsins D and O, alpha2-macroglobulin, alpha1-antitrypsin and PP5/TFPI-2 transcripts. Western blot analysis and immunoprecipitation experiments with cathepsin D and alpha1-antitrypsin antibodies indicated that metabolically labeled ciliary body explants and/or ciliary epithelial cells in vitro with 35S-methionine, synthesize and secrete these proteins. Cultured nonpigmented ciliary epithelial ODM-2 cells, in response to phorbol-12-myristate 13-acetate (PMA), but not to the non-protein kinase C binding phorbol ester 4 alpha-phorbol didecanoate (PDBu), elicited up-regulation (up to 5-fold) of transcription, synthesis and secretion of alpha1-antitrypsin. These results provide in vitro evidence that the ciliary epithelium synthesizes and secretes a selective group of proteinases and proteinase inhibitors detected also in aqueous humor. The expression of at least of one of the proteinase inhibitors, alpha1-antitrypsin, can be modulated in response to phorbol ester.

Ca2+ mobilization and interlayer signal transfer in the heterocellular bilayered epithelium of the rabbit ciliary body

1. 'Ratiometric' fura-2 methodology in slice preparations and 'intensitometric' fluo-3 measurements of confocal images were used to simultaneously monitor Ca2+ mobilization in the two distinct, apically joined cell layers which constitute the ciliary body epithelium (CBE): the non-pigmented (NPE) and pigmented (PE) epithelia. 2. Both methods yielded comparable results regarding Ca2+ responses in the syncytium upon stimulation with adrenergic and cholinergic agonists. 3. The alpha 1-adrenoceptor agonist phenylephrine elicited a moderate [Ca2+]i increase in the PE, whereas NPE [Ca2+]i remained unchanged or exhibited a slight diminution. 4. In combination with carbachol, the alpha 2-adrenoceptor agonist brimonidine elicited large Ca2+ increases (> 10-fold) in both the NPE and PE cell layers, even though previous studies indicated the absence of an alpha 2-adrenergic effect on [Ca2+]i in the PE. The onset, as well as the peak of the Ca2+ responses in PE cells frequently exhibited a small delay with respect to adjacent NPE cells. No such time difference was observed between adjacent NPE cells. 5. Pre-incubation of the ciliary body in Ca(2+)-free solution under conditions known to elicit overt NPE-PE separation abolished the alpha 2-adrenocholinergic response in the PE. 6. Addition of heptanol to the perfusate, to block gap-junctional communication, caused a small [Ca2+]i decrease in the NPE and a slight increase in PE[Ca2+]i. Subsequently, the Ca2+ mobilization in the Pe in response to the brimonidine and carbachol combination was either blocked or showed a substantial delay. The Ca2+ mobilization in the NPE, in contrast, remained unchanged. 7. We conclude that the heterocellular syncytium exhibits rectificatory behaviour with respect to Ca2+ mobilization; responses originating within the NPE are easily transferred to the PE, while the reverse does not occur.

Cell-attached patch clamping of the intact rabbit ciliary epithelium

Following thorough removal of adhering aqueous humor, we have succeeded in patch clamping the intact rabbit ciliary epithelium in the cell-attached and inside-out excised-patch modes. Rapidly fluctuating currents ("chatter activity') were observed during recordings conducted for as long as 1 h. Chatter activity did not reflect seal instability since interconversion was noted between chatter activity and transitions between stable open and closed states, excision of patches into the bath was associated with substantial shifts in the reversal potential, and chatter activity could be triggered by sustained hyperpolarization, but was insensitive to stretch. The chatter channel was identified as cation-nonselective from the reversal potentials both in the cell-attached and excised-patch modes. The channel's kinetics were similar to those of the cGMP-activated phototransduction channel. The results of PCR amplifications of fragments in cDNA libraries from both human ciliary body and human nonpigmented ciliary epithelial (NPE) cells indicated that human ciliary epithelial cells transcribe message for the retinal phototransduction channel. The possible role of the phototransduction channel in expressing chatter activity was further explored by perfusing preparations with a known activator of that channel (cGMP) and with a known inhibitor (L-cis-diltiazem). Neither agent significantly affected chatter behavior. We conclude that: (1) this is the first demonstration of the feasibility of patch-clamping the intact ciliary epithelium; (2) the NPE cells display chatter activity arising from rapidly fluctuating transitions of a cation-nonselective channel; (3) NPE cells can transcribe message for the cation-nonselective phototransduction channel; and (4) if the observed chatter activity is from a homologue of the photo-transduction channel, the homologue is pharmacologically distinct.

Inhibition of Na(+)-K(+)-ATPase activates Na(+)-K(+)-2Cl- cotransporter activity in cultured ciliary epithelium

Inhibition of Na(+)-K(+)-ATPase activates Na(+)-K(+)-2Cl- cotransporter activity in cultured ciliary epithelium. Am. J. Physiol. 266 (Cell Physiol. 35): C198-C205, 1994.--86Rb uptake experiments were conducted to measure Na(+)-K(+)-ATPase activity and Na(+)-K(+)-2Cl- cotransporter activity in a cell line derived from rabbit nonpigmented ciliary epithelium. The presence of a Na(+)-K(+)-2Cl- cotransporter was supported by the observation of a bumetanide-sensitive 86Rb uptake component that was dependent on the extracellular concentration of both sodium and chloride. Potassium influx mediated by the Na(+)-K(+)-2Cl- cotransporter and Na(+)-K(+)-ATPase accounted for approximately 46 and 33% of total potassium uptake, respectively, whereas both ouabain- and bumetanide-resistant uptake accounted for 9%. Inhibition of the Na(+)-K(+)-ATPase had a stimulatory effect on Na(+)-K(+)-2Cl- cotransporter activity, which was dependent on the extent and duration of Na(+)-K(+)-ATPase inhibition. Ouabain treatment stimulated the potassium (86Rb) efflux rate and reduced intracellular potassium ([K]i). Potassium channel blockers suppressed the ouabain-activated potassium efflux and inhibited the ouabain-induced activation of the Na(+)-K(+)-2Cl- cotransporter. We conclude that Na(+)-K(+)-ATPase inhibition leads to the opening of potassium channels, which exacerbates the depletion of cellular potassium; Na(+)-K(+)-2Cl- cotransporter stimulation caused by the fall of [K]i overrides the tendency of increased cellular sodium to inhibit the cotransporter.

Potassium transport in nonpigmented epithelial cells of ocular ciliary body: inhibition of a Na+, K+, Cl- cotransporter by protein kinase C

The mechanisms by which 86Rb+ (used as a tracer for K+) enters human nonpigmented ciliary epithelial cells were investigated. Ouabain-inhibitable bumetanide-insensitive 86Rb+ transport accounted for approximately 70-80% of total, whereas bumetanide-inhibitable ouabain-insensitive uptake accounted for 15-25% of total. K+ channel blockers such as BaCl2 reduced uptake by approximately 5%. Bumetanide inhibited 86Rb+ uptake with an IC50 of 0.5 microM, while furosemide inhibited with an IC50 of about 20 microM. Bumetanide-inhibitable 86Rb+ uptake was reduced in Na(+)-free or Cl(-)-free media, suggesting that Na+ and Cl- were required for optimal uptake via this mechanism. These characteristics are consistent with a Na+, K+, Cl- cotransporter in NPE cells. Treatment of NPE cells for 15 min with phorbol 12-myristate, 13-acetate (PMA), an activator of protein kinase C, caused a 50-70% decrease in 86Rb+ uptake via the Na+, K+, Cl- cotransporter. Other 86Rb+ uptake mechanisms were not affected. 86Rb+ uptake via the Na+, K+, Cl- cotransporter could be inhibited by other phorbol esters and by dioctanoylglycerol, an analog of diacylglycerol, but not by 4 alpha phorbol didecanoate, an ineffective activator of protein kinase C. Staurosporine, a protein kinase C inhibitor, blocked phorbol ester inhibition of 86Rb+ uptake. These data suggest that a Na+, K+, Cl- cotransporter in NPE cells is inhibited by activation of protein kinase C.

Effect of bicarbonate on intracellular potential of rabbit ciliary epithelium

Extracellular HCO3- hyperpolarizes the intracellular potential and makes the aqueous medium negative with respect to the stromal surface of the rabbit ciliary epithelial syncytium. The bases for these observations have been unclear. We have been studying the bicarbonate-induced hyperpolarization (BIH) with sustained intracellular recordings for periods as long as 1-2 hrs. The BIH was observed [6.0 +/- 0.4 mV (mean +/- SE, N = 22)] even when the external pH was clamped constant by appropriately changing the CO2 tension. External HCO3- was required since aeration with CO2 at low external pH did not replicate the BIH. DIDS [4,4'-diisothiocyano-2,2'-disulfonic acid] did not abolish the effect. The hyperpolarization is unlikely to reflect the pH dependence of K+ channels alone, since the effect was not reduced by either 2 mM Ba2+ alone or 2 mM Ba2+ together with 50-100 microM quinidine. The BIH depends directly or indirectly on external Na+, since the sign of the polarization response was reversed either by replacing Na+ with N-methyl-D-glucamine or by blocking the Na+,K(+)-exchange pump with 50-100 microM ouabain. Replacement of external Cl- with NO3- or application of the Cl(-)channel blocker NPPB [5-nitro-2-(3-phenylpropylamino)-benzoate] depolarized the membrane and reversed the sign of the BIH. The response of the ciliary epithelium to HCO3- is complex and may arise from several mechanisms. We suggest that one important element is an anion channel whose conductance is reduced by bicarbonate and whose reversal potential is indirectly dependent on the operations of the Na+,K(+)-pump and a Cl(-)-linked symport.

Immunocytochemical localization of carbonic anhydrase, NaK-ATPase and the bicarbonate chloride exchanger in the anterior segment of the human eye

We examined immunohistochemically the localization of three transport enzymes (carbonic anhydrase, Ca-II; sodium-potassium-activated adenosine triphosphatase, NaK-ATPase; bicarbonate-chloride exchanger, band III) in the anterior segment of the human eye. In accord with earlier studies, NaK-ATPase was primarily found in the corneal endothelium, but also in the corneal basal epithelial cell membranes. In addition, immunoreactivity for NaK-ATPase was observed in the non-pigmented epithelium of the ciliary processes and between the two epithelial cell layers. Ca-II immunoreactivity was found in the corneal endothelium as well as in the non-pigmented epithelial layer of the ciliary processes. Interestingly, band III immunoreactivity was found in the corneal endothelium, as similar to Ca-II, but not in the ciliary processes. These results show that, similar to many other tissues, Ca-II and band III immunoreactivities colocalize in the same cytologic site in the human corneal endothelium. Immunocytochemical detection of these key transport enzymes not only gives their accurate and reliable anatomical distribution, but also provides information on the electrolyte transport at these sites.

New developments in the drug treatment of glaucoma

This article reviews standard treatment modalities for patients with glaucoma and describes 3 classes of drugs which are undergoing development: apraclonidine (aplonidine, ALO 2145), an alpha 2-adrenergic agonist which has been released for clinical use; topical carbonic anhydrase inhibitors, a modification of the systemic carbonic anhydrase inhibitors currently in use; and prostaglandins (PGs), a new class of drugs with topical ocular hypotensive activity. Standard treatment modalities include parasympathomimetic agents such as pilocarpine, carbachol, and phospholine iodide, which lower intraocular pressure (IOP) by increasing aqueous outflow through the trabecular meshwork. A newer form of pilocarpine as a gel produces a longer action. Adrenergic agonist medications, such as epinephrine (adrenaline) and its prodrug dipivefrine (dipivalyl epinephrine), function by increasing uveoscleral outflow and trabecular outflow facility. A decrease in aqueous formation by the ciliary processes is thought to be the mechanism of action of beta-adrenoceptor antagonists, but the physiological basis for this action has not been clearly demonstrated. A newer beta-blocker, betaxolol, has relatively selective beta 1-blocking activity. Carbonic anhydrase inhibitors are nonbacteriostatic sulphonamide derivatives which decrease aqueous formation by the ciliary body. Almost 50% of patients taking these medications are unable to tolerate them because of their adverse effects, and there is thus much interest in the development of a topical carbonic anhydrase inhibitor with the potential for fewer adverse effects. MK 507 is the most recent and most potent compound in the series of topically active carbonic anhydrase inhibitors. Apraclonidine hydrochloride is a derivative of clonidine hydrochloride, an alpha 2-adrenergic agonist. Clonidine has previously been shown to lower IOP significantly, but has the potential to produce marked lowering of both systolic and diastolic blood pressures. Its major ocular effect appears to be a decrease in aqueous production. The structural modification to apraclonidine decreases corneal absorption and the drug's ability to cross the blood-brain barrier, minimising the risk of centrally mediated cardiovascular side effects. Apraclonidine may also influence secondary avenues of aqueous outflow, such as uveoscleral outflow, and may also affect conjunctival and episcleral vascular flow. It produces a mean decrease in IOP of 25% for as long as 12 hours. Adverse effects include blanching of the conjunctiva, minimal mydriasis and eyelid retraction. This drug has been approved in the US for use in prevention of elevated IOP after argon laser trabeculoplasty and iridotomy, and has potential uses in preventing an IOP rise after YAG laser posterior capsulotomy and cataract surgery in patients already on other antiglaucomatous medications.(ABSTRACT TRUNCATED AT 400 WORDS)

Ascorbate-stimulated active Na+ transport in rabbit ciliary epithelium

In a physiological medium (134 mM Na+ concentration), unidirectional blood-to-aqueous and aqueous-to-blood Na+ fluxes across the isolated rabbit ciliary epithelium are large, rendering the detection of a net transport difficult. At 134 mM an active component for Na+ may be obscured by diffusional fluxes and a bidirectional Na(+)-Cl- cotransport. Considering that the active transport saturates at about 30 mM, experiments were performed at this reduced Na+ concentration to minimize the influence of diffusional pathways. A net blood-to-aqueous Na+ flux that ranged from 0.25 to 0.81 mu eq/hr was obtained. Addition of ascorbic acid to the aqueous side under this condition increased the blood-to-aqueous flux with little effect on the flux in the opposite direction. Ouabain inhibited both the Na+ and ascorbate-stimulated Na+ transport. The increase in blood-to-aqueous Na+ flux by ascorbate was also observed in tissues bathed with [Na+] closer to physiological levels (100 mM). These results indicate that the rabbit ciliary epithelium transports Na+ into the posterior chamber. Since aqueous ascorbate stimulates Na+ transport, it may be implicated in both Na+ movement and aqueous humor secretion. However, the rate of Na+ transport can only account for a small fraction of total aqueous humor production.

Induction of alkoxyresorufin O-dealkylases and UDP-glucuronosyl transferase by phenobarbital and 3-methylcholanthrene in primary cultures of porcine ciliary epithelial cells

Our previous studies have shown that drug-metabolizing activities in the eye are highest in the ciliary body, a tissue responsible for aqueous humor production. In this work, we have separated nonpigmented epithelial (NPE) cells and pigmented epithelial (PE) cells from porcine ciliary body and determined basal and induced activities of 7-ethoxyresorufin (ER) O-dealkylase, 7-pentoxyresorufin (PR) O-dealkylase, and UDP-glucuronosyl transferase (UDP-GT) using primary cultures of separated cells. ER and PR activities were associated primarily with NPE cells and were very low in PE cells. Treatment of NPE cells with phenobarbital (PB) for 48 hr resulted in about a 4-fold increase in PR O-dealkylase activity but only a 1.3-fold rise in ER O-dealkylase activity. Conversely, 3-methylcholanthrene (MC) treatment augmented the ER O-dealkylase activity of NPE cells 6 times over the basal activity in 48 hr but had little effect on PR O-dealkylase activity. Both NPE and PE cells had low basal UDP-GT activities. UDP-GT activity increased about 5-fold in PB-treated PE cells and about 4-fold in PB-treated NPE cells in 48 hr. The results of MC treatment were similar to those of PB treatment; enhancement of UDP-GT was more pronounced in PE cells than in NPE cells. Induction by PB and MC of ER O-dealkylase, PR O-dealkylase and UDP-GT activities in ciliary NPE and PE cells was inhibited almost completely by 3.5 microM cyclohexamide and 40 nM actinomycin D. The heterogeneous distribution of these enzymes suggests that a harmonious interplay between NPE and PE cells is important for metabolic detoxification of blood plasma prior to aqueous humor formation.